Britain must be prepared to fight a war in three years’ time and double the lethality of its army as the separate threats of Russia, China, Iran and North Korea come to a head, the new chief of the army has warned.

Gen Sir Roly Walker, the chief of the general staff, told reporters that the west faced “an axis of upheaval” with increasing military ambition and that a conflict involving one of the countries could lead to “a significant detonation” in another theatre.

The UK and its allies had to be ready “to deter or fight a war in three years”, he argued – a deliberately stark judgment based on China’s hostility towards Taiwan, Iran’s nuclear ambitions and Russia’s militarisation as demonstrated by its invasion of Ukraine.

Walker cited US assessments that China’s president, Xi Jinping, had asked for the country’s military to be ready to invade Taiwan in 2027 – as well as concerns that Iran could seek to break the nuclear deal that is intended to prevent it developing an atomic weapon, while the war in Ukraine is continuing.

In a subsequent speech, Walker said he had “a bold ambition” for the army “to double our fighting power in three years and triple by the end of the decade”, not with extra resources but by using technology and techniques developed on the battlefields of Ukraine, such as drones and artificial intelligence.

Walker did not set out the nature of the global threats in the main body of his address, but in a briefing to reporters he argued that the interdependence between Russia, China, Iran and North Korea was growing, as seen in Ukraine, where Iran and North Korea have supplied weapons and China components to aid Moscow’s military effort.

“The problem is, the ability to manage any one crisis that involves any of those actors becomes much, much harder,” because they are increasingly supporting each other with weapons, components and intelligence, the general said.

Although Russia was embroiled in the war in Ukraine, and on current trends its forces would, Walker said, take “five years to grind their way through” to capturing the eastern Donbas, at a cost of 1.5 million casualties, the country’s history had shown it could bounce back.

In his earlier remarks to reporters, the army chief argued Russia could emerge from its current war with “a sense of wanting retribution for the support that was given to Ukraine” – and could pose a greater medium-term threat than anticipated.

Walker argued that “we are not on an inexorable path to war” but added that “what we do have is an absolute urgency to restore credible hard power in order to underwrite deterrence”, at a time when the Labour government has just begun a strategic defence review after the election.

A week ago, the man leading the review, the former Nato secretary general George Robertson, warned that Britain and its allies were facing a “deadly quartet” of China, Russia, Iran and North Korea, suggesting the idea that the four countries were increasingly collaborating was widely shared in security circles.

The new army chief’s comments come at a time when the size of the British army is at its smallest for 300 years, having donated tanks, missiles, artillery and large amounts of ammunition to Ukraine – and will be interpreted by some as a plea for future investment in a service considered in need of modernisation.

While the Labour government has agreed to lift defence spending from the current level of 2.32% of GDP – £64.6bn – to 2.5%, it has not yet committed to a firm timetable. Ministers want to look at the country’s books before deciding what is possible, while the Conservatives previously committed to doing so by 2030.

Though Walker was not immediately asking for more money, earlier his boss Adm Sir Tony Radakin, the chief of defence staff, said the British army could justify extra investment if its effectiveness improved.

“Politics responds to positive reasons to invest,” Radakin told the land warfare conference, and argued that the more “the ambition grows”, the more the military was able to “make the case for a stronger and more capable army”.

Radakin did not call for a return to national service, a surprise election promise made by the defeated Conservatives, and said the UK’s national security did not depend on an army enlarged by young conscripts.

“Poland is doubling the size of its army over the next decade. The Baltic and Nordic states are talking about mass resilience and conscription. That is understandable. They border Russia. The threat is close. Our geography is different,” Radakin said.

But Walker said it was a concern that Britons did not appear as willing to fight for their country as those further to the east in Europe. “The problem is that we do not think we have a problem,” he said.